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November, 1945 


L. D. BATCHELOB, 2 0. L. BRAUCHER, 3 and E. F. SERE 4 

The Persian walnut (Juglans regia), commonly known as the English wal- 
nut, is grown in nearly every county in the state of California. The center of 
walnut production, however, has persistently moved northward during the 
past two decades. The following eight counties produced more than two thirds 
of the state's crops in 1943 and 1944, and are mentioned in the approximate 
order of their importance : Ventura, San Joaquin, Contra Costa, Santa Clara, 
Los Angeles, Tulare, Stanislaus, and Riverside. 

In the state about 126,000 acres of walnut trees produce annually 55,000 to 
65,000 tons of orchard-run nuts. There is a marked fluctuation in production 
from year to year. The average orchard-run production of the four-year 
period, 1915 to 1918, inclusive, was approximately 20,000 tons; the average 
during 1931 to 1934, inclusive, was 37,150 tons. 

Importations of foreign walnuts to this country increased up through the 
year 1924. A marked decrease occurred in succeeding years, the importations 
ceasing entirely during World War II. This drop was due to import duties, to 
economic conditions, to an increasing preference for the domestic walnut, and 
to a domestic surplus which accumulated during the years 1930 to 1940. Dur- 
ing the period 1939 to 1944, approximately 91 per cent of the entire crop 
produced in the United States was grown in California. 

A walnut orchard may be expected to bear commercial crops by the time the 
trees are six to ten years old, according to the variety, the number of trees per 
acre, and such conditions as soil fertility, irrigation practice, and climate. 

The profitable life of a walnut orchard cannot be determined from experi- 
ences in California. With good soil, favorable climatic and water conditions, 
walnut trees will produce satisfactory crops for a long period. Orchards sixty 
years old are among the most productive in the state. 


The chief climatic limitations of the walnuts are frosts in spring and fall, 
extreme heat in summer, and insufficient winter chilling. 

1 This circular supersedes Bulletin 379, Walnut Culture in California, by L. D. Batchelor. 

2 Horticulturist in the Experiment Station. 

8 Assistant Field Manager, California Walnut Growers Association. 
* Specialist in Agricultural Extension. 


2 California Experiment Station Circular 364 

Frost Injury. — Frosty areas should be avoided for walnuts. Spring frosts 
below 30° F will injure catkins, new growth, and young nuts. Late-blooming 
varieties are less subject to injury from the spring frosts than the earlier- 
blooming types. When frosts occur as late as May, the late-blooming varieties 
may suffer more injury, since the nuts (i/ 2 inch or more in diameter) of the 
early-blooming varieties will stand lower temperatures at this time than the 
blossoms of the late varieties. 

Young twig growth is subject to injury by early fall frosts. The injury may 
not be apparent until spring, when the tree fails to leaf out on the injured part 
of the past season's growth. Dormant trees are seldom injured by winter tem- 
peratures occurring in California walnut districts. 

The French varieties Mayette and Franquette have grown without severe 
injury where the minimum winter temperature occasionally reaches 0° F. 

Heat Injury. — Temperatures above 100° F, accompanied by low humidity, 
cause walnuts to sunburn if they are exposed to the direct rays of the sun. In 
the inland valleys this temperature, or higher, often occurs during the summer 
months. Sunburned nuts usually become "blanks" if the injury«occurs during 
June or July. If the sunburning occurs when the nuts are more fully developed 
but not mature, the kernel may become partially shriveled. High temperatures 
cause a large percentage of the kernels to darken in color. A sunburned hull 
may stick to the shell or stain it, causing the nut to be a cull. 

Winter Chilling Requirements. — In spring seasons following very warm 
winters, especially in warm coastal locations of southern California, walnuts 
will be delayed in starting growth. In such years, walnuts produced on shoots 
growing from buds which start growth very late will be undersized, and the 
total crop sometimes will be materially reduced. Most French varieties, such 
as the Franquette and Mayette, have a greater winter chilling requirement 
than the Santa Barbara soft-shell types. These French varieties are not suited 
to southern California conditions, although they are satisfactory in central 
and northern California where winters are cooler. 


Depth and Character of Soil. — Successful walnut culture is dependent upon 
favorable soil conditions. A well-drained silt loam soil at least 5 to 6 feet deep, 
containing abundant organic matter, free from a high or fluctuating water 
table, and free from alkali, is ideal. There are moderately successful orchards 
on fine silt soil underlain with sand within 4 to 5 feet of the surface, but such 
properties require skill in management. Coarse sandy soils, heavy adobe soils, 
and clay loams underlain by adobe are not well suited to walnut orchards. 

Drainage and Alkali Injury. — Most of the high water tables in arid regions 
carry more or less alkali in solution. The mere presence of a water table within 
9 or 10 feet of the surface is usually only part of the difficulty — the alkali being 
of greater concern — for the deep-rooted trees obtain a portion of their mois- 
ture from this water table, and may be injured by the salts in it, even though 
there is a seemingly sufficient layer of good soil above the water table. 

It is difficult to place a safe boundary beyond which injury from a high 
water table is not likely to occur. As much depends upon the nature of the 
water as upon the actual depth below the surface of the ground. The success of 

Walnut Production in California 3 

walnut culture on land with a high water table is influenced by the rainfall, 
irrigation practice, nature of the soil, and quality of the irrigation water. 
Orchards severely injured by an alkali water table have improved to some 
degree after the installation of drains. 

Quality of Irrigation Water Used. — The quality of irrigation water for wal- 
nuts is important. Relatively small amounts of alkali salts in the water are 
harmful to walnut trees. Of all the orchard crops in California, walnuts seem 
to be one of the most sensitive to alkali injury. In districts of heavy rainfall 
and on well-drained soils, water of poor quality may be used with less harm 
to trees than in the more arid regions, or on soils which are poorly drained. 

The kind of salts contained in the water bear as greatly upon the injury as 
do the type of soil and the other conditions of the orchard. Even a small 
amount of boron in the irrigation water causes marginal leaf scorch and some- 
times severe defoliation, with consequent lowering of quality and yields. Any 
new water supply should be analyzed, and an expert opinion given regarding 
its suitability for walnut culture. 

When common salt is the predominating toxic material, injury is apparent 
during the latter part of the summer or early fall. The point of the leaf and 
margins, and finally most of the leaf surface turn brown, causing a premature 
leaf drop. Severe injury causes the leaves to drop as early as August. This may 
result in an abnormal leafing out and blossoming in late summer or fall. 

Rainfall. — The amount of rainfall is a good indication of the amount of irri- 
gation water necessary to supplement it. Other factors, such as frequency, 
duration, amount of individual rains, runoff, and season of occurrence, must 
also be considered, since they influence the proportion of the total precipita- 
tion which penetrates into the root zone of the walnut trees. 

In many areas the rainfall is not sufficient to wet the soil to a depth of 6 feet 
even when no covercrop is grown. Some districts receive enough to grow a good 
covercrop, but the soil is dry by late winter. A few areas receive enough rain- 
fall to wet the soil to a depth of 6 feet and also to support a winter covercrop. 
Winter irrigation is advised in those areas where rainfall will not supply the 
soil moisture needed to produce a covercrop and penetrate to a depth of 6 feet. 


History. — The original walnut plantings in California were the hard-shell 
type of seedling trees, few of which remain. 

In 1867, near Santa Barbara, Joseph Sexton planted part of a sack of wal- 
nuts which were probably imported from Chile. The resultant trees produced 
both hard-shell and paper-shell types of nuts. Second-generation trees from 
nuts of this planting produced the Santa Barbara soft-shell. The walnut in- 
dustry in southern California developed from seedlings grown from the best 
Santa Barbara soft-shell trees. In southern California, practically all varieties 
(except the Payne and Eureka) are descendants of the original Sexton plant- 
ing. Such varieties include the Placentia, Pride of Ventura, Neff, Prolific, 
Wasson, Ehrhardt, and Chase. 

Many of the varieties of walnuts grown in France were imported into north- 
ern California about 1870 by Felix Gillet. The Franquette, one of the most 
valuable varieties grown in France, has become the leading variety of central 

4 California Experiment Station Circular 364 

California. The Mayette is also another imported French variety. A seedling 
from the Gillet nursery produced the original Concord tree in Contra Costa 
County. The Payne variety, a seedling, probably traces its heritage to the 
French importations. 

New varieties which would combine the best individual characters of present 
varieties and be better adapted to special climatic conditions are needed. A 
number of promising seedlings are being tested now. Individuals contemplat- 
ing new plantings should discuss the variety situation in their districts with 
local representatives of the College of Agriculture, packing-house managers, 
and experienced growers. 

Choice of a Variety. — The varieties chosen for planting should be those 
which experience has shown to produce the highest yields and best quality of 
nuts. New varieties of the future should be a decided improvement upon the 
present varieties ; otherwise they are not worthy of propagation. 

It is essential to have nuts with a strong, well-sealed shell to withstand han- 
dling in packing and shipping without cracking. The proportion of kernel to 
the total weight of the nut in the leading varieties varies between 40 and 50 
per cent. 

No one walnut variety is suited for planting throughout the whole state. The 
success or failure of a variety depends upon its adaptation to its surroundings. 

Placentia. — The Placentia is one of the favorite varieties in southern Cali- 
fornia. The young trees grow rapidly, are precocious yielders, and have a 
tendency to bear good crops annually. In some areas, however, the nuts blight 
badly. Another defect of the Placentia is its tendency to spring open at the 
apex if dried too rapidly. The nuts are of desirable size. They have a fairly 
smooth shell and usually are oval, though they tend to vary in shape, some 
strains being nearly round and somewhat roughened. The shell is thin, but 
strong. The kernel is smooth, plump, and light colored, and is taken as a stand- 
ard of quality and appearance for the Budded grade of nuts. The kernel qual- 
ity is usually poorer in the interior valleys of southern California than in the 
coastal areas. The Placentia is not satisfactory in central or northern Cali- 

Eureka. — The Eureka tree grows vigorously but comes into bearing at a 
later age than the Placentia or the Payne. It blooms rather late and generally 
escapes injury by spring frosts. The nuts ripen three weeks later than those 
of the Placentia. The Eureka is distinguished from other varieties by its pro- 
nounced elongation, rather straight, parallel sides, slightly rounding to square 
ends. Its chief defect is a condition known as "shrivel tip" of the kernels. This 
trouble is especially severe in the inland districts. 

Blackmer. — This variety is also known as : Mautner, Westf all, Westphal, 
Meridian, Leib and Leib Special, Vaughn's Pride, and Money Maker. The 
Blackmer so closely resembles the Eureka that they are often packed together. 

Ehrhardt. — The Ehrhardt walnut is a Santa Barbara soft-shell type, closely 
resembling the Placentia. It is somewhat rougher, slightly larger, and better 

Wasson. — The Wasson is medium sized and rather rough ; the shell firm and 
well sealed. It is a Santa Barbara soft-shell type and bears relatively large 
crops. The Wasson is adapted to the same sections as the Placentia. 

Walnut Production in California 5 

Pride of Ventura. — The Pride of Ventura is a selected type of Santa Bar- 
bara soft-shell seedling and is grown in southern California. Its alternate 
bearing habit is its main defect. The nuts are large and fairly smooth, the shell 
is heavy and well sealed. The kernel quality is superior to that of the Placentia. 

Franquette. — The Franquette blooms about four weeks later than the Payne 
and later than the Eureka. Thus it escapes injury by spring frosts. It is slow 
to reach full bearing. 

The nut is elongated, pointed, and moderately rough. The shell is very well 
sealed, and fairly well filled with a light-colored kernel. The Franquette is 
recommended for central and northern California. 

Although the true Franquette is a distinct clonal variety which originated 
in France, it has been propagated in California partly from seedlings. Here 
the name Franquette now includes a group of similar strains or varieties mar- 
keted as Franquette but varying in habits of growth, yields, etc. Some strains 
are very slow in coming into bearing and are poor yielders. Therefore, when 
planting Franquette nursery trees or topworking to Franquette, it is very 
desirable to be sure that the scions or budwood come from trees of a strain 
which bears good crops of high quality in the district. 

Mayette.— The name Mayette, as used in California, includes a large group 
of somewhat similar strains or varieties. The trees are large and spreading and 
start spring growth about two weeks before the Franquette. The strains may be 
further classified as the round type, such as the San Jose Mayette, the round, 
hard-shell type, such as the Triple X, and the long type, such as the Tribble. In 
most Mayettes the meat is relatively small in proportion to the size of the shell, 
but the color is good. The San Jose Mayette is very poorly sealed and requires 
packing in cartons. The Triple X has a heavy shell and is well sealed but has 
not yielded consistently large crops. It holds its catkins very late and is some- 
times used as a pollinizer for the Franquette. Another strain, the Tribble, 
yields high in some districts and is well sealed. Mayettes are grown only in cen- 
tral and northern California. 

Hartley. — This is a seedling of French parentage, selected by John Hart- 
ley of Napa, which has been propagated widely in central and northern 
California during the past ten years. In tree habit it resembles the Mayette 
but is a weaker grower than the usual Mayette. The nuts are longer and more 
pointed than most Mayettes. Quality is high and sizes are good. The trees come 
into bearing very early and produce large crops under favorable growing con- 

Payne. — The parent seedling tree of the Payne variety was discovered by 
G-. P. Payne, near Campbell, California. This variety is now grown in most 
walnut areas. Its early and heavy bearing are its chief desirable characteristics. 
The Payne tree, because of its very heavy production as a young tree, makes a 
slow growth. The nuts are borne prominently on the outside of the trees and 
are subject to sunburning. The shell is of medium thickness, somewhat pitted, 
and well sealed ; the kernel is full, with moderate convolutions, and of good 

The Payne is a desirable variety to interplant in a Franquette orchard, and 
for such use it may be of great value. Thus, during the first ten or twelve years 
of the growth of the orchard, the total production will be more than doubled. 

6 California Experiment Station Circular 364 

After this period the Paynes should be removed because the Franquette trees 
will require all of the space. 

The Payne is notoriously subject to blight and, until satisfactory control 
measures are known, cannot be recommended as a permanent tree in those 
areas where blight is regularly prevalent and severe. 

Concord. — The Concord tree is vigorous and an annual producer of medium- 
sized crops. The shell is rather smooth; the nuts are round type and well 
sealed ; the kernel is fairly plump, medium light colored, and of good quality. 
This variety is not recommended for planting except in central coast counties. 
Its kernel quality is extremely poor in the inland valleys. 


Northern California Black Walnut. — The choice of rootstocks for the wal- 
nut has narrowed down to a preference for the northern California black wal- 
nut because it makes a good graft union and has certain other advantages. 
This species is resistant to some degree to the oak root fungus (armillaria root 
rot) . It is apparently resistant to the common root-knot nematode, Heterodera 
marioni (Cornu), and the nematode Cacopaurus pestis, but is sometimes in- 
jured by the meadow nematode, Pratylenchus pratensis (de Man). 

The greatest disadvantage of the northern California black walnut as a root- 
stock is its susceptibility to crown rot. The matter is further complicated by 
the fact that the northern California black walnut is often confused with the 
southern California black walnut, which is especially susceptible to both crown 
rot and root rot and is therefore not recommended as a rootstock. Plantings 
of both species occur in southern and northern California. The fact that the 
seed came from northern California has not always been an assurance that the 
northern species was obtained. 

Hybrid Rootstocks. — The Paradox-hybrid rootstock, which is a cross be- 
tween the English and any of the black walnuts, makes a rapid-growing tree. 
Such trees have not been produced in any large quantity in the past, and 
because of the difficulties in propagating hybrids it seems probable that their 
use in the future will be restricted to areas where the added vigor of the hybrid 
is of especial importance. 

The Royal-hybrid walnut is a cross between the eastern black walnut and 
either one or the other of the California black walnuts. Its use as a rootstock 
is limited by the difficulty in producing the hybrid seed stock. 

Persian Walnidt Seedlings. — The Persian walnut seedling grown from espe- 
cially vigorous trees, such as a hard-shell variety, is a good rootstock wherever 
oak root fungus or alkali soil is not a consideration. These seedlings have the 
advantage of making a smooth graft union free from any constriction. They 
are also apparently much more resistant, under orchard conditions, to crown 
rot and to root rot. These two characteristics of the Persian walnut have been 
the principal reason for its popularity in several of the districts of southern 
California. The Persian walnut stock is more subject to injury by the common 
root-knot nematode than is the northern California black. In one district 
Persian seedling trees are being killed apparently by the nematode Cacopaurus 
pestis. The slow initial growth of the Persian stock is objectionable to the 

Walnut Production in California 7 


Walnut trees can be propagated in the nursery either by grafting or bud- 
ding. Most southern California nurserymen prefer grafting, whereas in north- 
ern California budding is the usual practice. High budding gives a short 
section of black walnut trunk. This decreases danger of sunburn and prevents 
entrance of oak root fungus into the Persian trunk at the ground line. 

Grafting. — One year after planting the nuts, the seedlings should be 1 
inch or more in diameter at the surface of the ground. The surface soil is hoed 
away from the crown of the trees to a depth of 2 to 3 inches, and the scion is 
inserted in the stock just below the level of the ground. After the scion is tied 
and thoroughly waxed, the soil is hoed back, covering the scion to a depth of 
1 to 2 inches. 

The grafted trees are grown one year in the nursery and trained to a whip- 
like growth free from lateral branches. Each tree must be tied to a stake ap- 
proximately 1x2 inches x 8 feet. 

Budding. — When walnuts are propagated by budding, the nuts are given 
an early start and the seedlings are kept growing rapidly until August. At 
this season they are large enough to receive a bud near the surface of the 
ground. Patch budding is the method most commonly followed, although shield 
budding may be successfully used if the chip of wood is carefully removed 
from the bud. The bud should be ripened by removing the leaf, leaving the 
petiole attached to the base of the bud for a period of about 10 days before 
cutting. After the bud is inserted in the stock, it is firmly tied in place by means 
of waxed cloth. Ties on buds applied during the growing season must be loos- 
ened at about 10-day intervals or at least often enough to prevent severe con- 
striction. Late summer or fall buds are allowed to remain dormant until the 
following spring. 

Requirements for Good Nursery Trees. — Grafted trees sell according to size, 
with a premium placed upon the larger trees. A medium-sized tree (8 to 10 feet) 
may be preferable to either an extremely large, or a very small tree. A very 
large tree may be injured considerably in the process of digging it from the 
nursery plot ; a small one may be stunted, with a poor root system, and may 
never make a first-class orchard tree. 


Arrangement of ihe Orchard. — Walnut trees are spaced so as to permit the 
planting of 12 to 27 per acre in mature orchards, the number depending on 
variety and growing conditions. Mature orchards grown under favorable con- 
ditions with trees 60 feet apart each way are among the most productive in 
the state. With this spacing, individual trees haA^e room to develop fully. A 
large proportion of the nuts are produced on the side branches, and the trees 
maintain a healthy growth of new fruiting wood for many years. 

In close plantings — trees 30 to 40 feet apart — the side branches are shaded, 
the fruit spurs on the lower branches die, and the crop is borne mainly in the 
tops. Plantings, 30 to 40 feet apart, should be thinned out by removing every 
other diagonal row (fig. 1) . The remaining trees will then stand approximately 
42 to 56 feet apart. 


California Experiment Station Circular 364 

One of the most favored systems is to plant the orchard 30 by 30 feet. The 
trees may be all of one variety, or of two, planted alternately. With the latter 
method, there is the advantage of the choice between two varieties when the 
time arrives for removing half of the trees. If this method is carried out, nearly 
twice the tonnage may be expected during the first ten or fourteen years than 
if only the permanent trees are planted. Orchard thinning is usually started 
when the trees are from ten to fourteen years old, the time varying according 
to tree size. Trees should not be left until they crowd excessively, as illustrated 
in figure 2. 

Fig. 1. — An old seedling orchard thinned out by removing every 
other diagonal row. (From Bui. 379.) 

Care of Trees before Planting. — If the trees are received from the nursery 
before the ground is ready for planting, they should be unpacked and heeled 
in where they will be shaded. The soil around the roots should be thoroughly 
watered after they are heeled in. If it is more practical, the trees, with the roots 
packed in damp sawdust or shavings, may be held for a time under a shed. 

Planting Nursery Trees. — Walnut trees should be planted during January 
or February so that the soil may be thoroughly settled around the roots and 
growth may start with the beginning of the normal growing season. Holes 
should be dug deep enough to allow room for the full length of the taproot, 
which may be from 18 to 30 inches. The lateral roots may be 6 to 8 inches long, 
and the hole should be wide enough to accommodate them. 

In filling in the soil around the roots, the topsoil should be used. It should 
be tamped thoroughly without bruising the roots. The young trees should be 
irrigated as soon as they are planted. 

Planting the Orchard with Black Walnut Trees, and Topworking. — The 
practice of topworking black walnut trees planted in place is common in cen- 
tral and northern California. Its justification is based on a greater percentage 
of survival and superior vigor of black seedlings as compared with Persian 

Walnut Production in California 9 

walnut trees propagated in nurseries under the somewhat unfavorable condi- 
tions that often occur in hot interior valleys, unirrigated mountain districts, 
or when intercrops are grown close to the trees. Young black walnuts can bet- 
ter withstand water shortages and competition from intercrops and weeds ; 
they also are more resistant to sunburn and borer attacks. Top working in the 
orchard requires considerable skill and consistent follow-up care. 

The planting of one-year-old black walnut trees is preferred to the method 
of planting the black walnut seed in place, although both methods are used. 
The nursery-grown black seedlings are more easily cared for and usually give 

■ i -j^* ■ : K 


mmk "m 

Fig. 2. — A typical example of crowded trees, being spaced 40 feet. They are pro- 
ducing walnuts only in the tops, and some trees will soon have to be removed in 
order to obtain a satisfactory yield per acre. (From Bui. 332.) 

a more uniform stand. Only vigorous trees should be planted. There is evi- 
dently no disadvantage in cutting the taproot since a better branched root sys- 
tem may result. 

When the black walnuts are established by planting the nuts in the field, two 
to four walnuts are planted in each place where a tree is to grow. Later the 
smaller trees are pulled out, leaving the most vigorous. 

Black walnut seedlings are usually large enough to bud during the second 
summer or to graft the following winter. Topworking when the trees are young 
decreases the need of propping and tying. Trees can be topworked at a single 
point on the trunk, or buds or grafts can be placed in side branches. The latter 
method gives a tree with multiple unions and main crotches of black walnut 
wood. Such trees are somewhat stronger and less likely to split, although sin- 
gle-union trees are generally satisfactory if main limbs are properly spaced 
on the trunk. 

Where budding is practiced the patch bud is most commonly used. Budding 
can be done whenever the bark on the black walnuts "slips" well and suitable 
buds are available. Budding is usually done in summer (July-August) and 
spring (April-May) . Spring buds are forced into growth by cutting back the 


California Experiment Station Circular 364 

stocks as soon as the buds are established. Summer buds are left dormant until 
the following spring. 

Grafting is done in late winter or early spring. Best success is usually ob- 
tained when grafts are placed just after growth starts on the stocks. Methods 
depend on the size of stock to be topworked and the experience of the opera- 
tor. Side grafts and whip grafts are best for small branches, those of % to 1 
inch diameter. Bark, cleft, or saw-kerf grafts are better for stubs 3 to 5 inches 
in diameter. To decrease danger of serious heart-rot infections, it is desirable 
to graft large trees high enough so that stubs are not larger than about 4 to 6 


Fig. 3. — Bark grafting, showing typical scions, with method of inserting them in the 
stock and holding them secure by small nails. 

inches in diameter. Bark-grafting methods illustrated by figure 3 are generally 
the most successful although other methods are sometimes used by highly 
skilled workers. 

Whatever method of grafting is practiced, waxing to prevent drying out is 
one of the most important details. All cut surfaces and bark cracks of the stock 
and scion should be covered with grafting wax as soon as the scions have been 
tied or nailed in place. Frequent inspections and rewaxing to keep all cracks 
covered are essential. Water emulsified asphalt compounds make a satisfac- 
tory grafting wax and can be used cold. If grafts are tied in with strings, these 
should be cut 3 weeks or more after growth starts, before they tighten enough 
to restrict growth. Covering grafts with punctured paper bags seems desirable. 

Whatever method of top working is practiced, bracing and tying of the new 
growth, as illustrated by figure 4, is usually necessary to prevent breakage 
during the first and second years. It is not advisable to let the growth go be- 

Walnut Production in California 


Fig. 4. — Supports in place on which to tie growing scions on top- 
worked trees during the first year. (From Bui. 379.) 

12 California Experiment Station Circular 364 

yond 12 or 15 inches before the supports are put in place and the first tying is 
done. Additional ties should be placed as needed throughout the first growing 

All water sprouts from the stock starting within 6 to 10 inches of the scions 
or buds should be removed completely at frequent intervals during the first 
spring and summer to prevent excessive competition. In cool to intermediate 
districts water sprouts farther down on the limbs or trunk can be removed 
completely before they reach 6 inches in length. In hot districts they can be 
slashed back to about 6 inches whenever they reach a length of about 1 foot. 
The}^ should not be allowed to grow to considerable size and then removed com- 
pletely, especially in hot interior districts, until the new top is well established. 
Complete removal during midsummer often results in severe sunburn injury 
to the trunks. 

Training Young Nursery-grown Trees. — Walnut trees come from the nur- 
sery as one-year-old whips. In cool coastal areas where sunburn can be pre- 
vented easily by whitewashing or by using tree protectors, the trees are usually 
headed from 5 to 6 feet above the ground at planting time Here, buds can be 
rubbed off the lower part of the trunk, or growth from them removed as soon 
as it starts. In this way, higher buds will be forced into growth to form part 
of the main framework of the tree. 

In hot interior districts, where danger of trunk sunburn is more serious, 
trees can be cut back more severely at planting, with only 5 to 7 buds left above 
the rootstock union (usually 18 inches) . This forces the low buds into growth, 
which will protect more effectively the lower part of the trunk from sunburn. 
At the time of the first winter pruning the most vigorous upright branch is 
selected to form the trunk of the future tree and is not cut back. One or two 
small lower branches on the southwest or south side can be cut back to short 
stubs and left for one year to aid in sunburn protection. All other branches 
should be removed. This low heading at planting time should not determine 
the height of main framework branches. At the end of the first year the most 
vigorous and upright branch may be selected to take the place of the cut off 
top and continue the leader form of tree. Other branches should be removed or 
cut back. If a vase-shaped tree with a high head is wanted, the central branch 
can be cut back after it has passed a desirable height and lateral buds will then 
develop into main framework branches. 

Trees are usually trained in one of three forms : central leader, modified 
leader, or vase shape. The advantage of the leader type is primarily in the 
greater strength of framework, there being many more lateral limbs distrib- 
uted along a greater space on the main trunk. The laterals should be spaced 
preferably about 2 feet apart vertically and spiraled around the trunk so that 
the horizontal angle between adjacent branches is about one third of a circle. 
A modified leader type is started like a full central leader, but the leader is lost 
after development of three to six main framework branches. In the vase type 
there are usually several main framework branches, originating at nearly the 
same point on the main trunk. This centralizes the strain of supporting the top. 
Vase-shaped trees, being weak at the crotch, are subject to more breakage. 
Many vase trees have too many main branches. Not more than three or four are 

Walnut Production in California 13 

Development of central-leader or modified-leader-type trees requires more 
attention and careful pruning than the more common vase shape. It is a fairly 
easy process with upright growers like Eureka and Franquette, but difficult 
with more naturally spreading trees like Placentia, Payne, and Mayette. 

Whether the ideal selection is the full central leader, the modified leader, or 
the open-vase type, the permanent branches to form the framework of the trees 
should be selected so that they are spaced as far apart as practicable, both ver- 
tically and horizontally. Excess branches should be removed at winter pruning 
time during the first several years, but in interior districts a few small, low 
branches on the southwest side of the trunk should be stubbed back and left 
for shade until the top develops sufficiently to protect the trunk. 

The first main framework branch should not be lower than 4^2-6 feet from 
the ground. Secondary laterals tend to become horizontal as the trees mature 
and should not be started lower than 8 feet from the ground, except under un- 
usual conditions of very strong prevailing winds or where certain varieties, 
such as the Payne, do not make large trees. 

It should be remembered that several years are usually required to develop 
a satisfactory framework. Young walnut trees should be pruned consistently 
each j^ear with a definite type of framework in mind. 


Walnut trees are naturally deep-rooted, and the character of surface-soil 
cultivation is of minor importance compared to the welfare of the tree. 

Cultivation of any kind — plowing, disking, harrowing, or any other method 
of working up the surface soil — is only a means to an end. In itself, cultivation 
is not a practice essential to the well-being of the walnut orchard. 

Purposes of Cultivation. — Cultivation of the soil in a walnut orchard is 
practiced to achieve the following results : 

1. To incorporate into the surface soil covercrops, green manures, fertilizers, 
and bulky organic matter of any kind. 

2. To keep summer weed growth down to a point where the weeds do not 
compete with the tree for the available supply of soil moisture. The amount 
required and the cost and supply of irrigation water, as compared with culti- 
vation costs, should be the determining factor in deciding whether clean cul- 
ture or "weedy culture" is to be followed. 

3. To keep the surface soil in such condition as to permit preparation of 
irrigation furrows, dikes, or basins. Efficient irrigation is largely dependent 
upon the distribution of irrigation water to where it is needed. 

4. To prepare a smooth soil surface so the nuts can be readily seen and har- 
vested efficiently. 

5. To prepare a good seed bed for planting the covercrop and to make the 
land ready for subsequent irrigation. 

6. To put the surface soil in a condition to retain a maximum amount of the 
winter rainfall with a minimum amount of soil erosion. 

Injury from Improper Cultivation Practices. — The practice of subsoiling 
or deep tillage of any kind in walnut orchards is never justified. Repeated cul- 
tivation of wet soil will pack it, and form a plowsole which will make water 
penetration difficult. Excessive cultivation of dry soil also injures its structure. 

14 California Experiment Station Circular 364 


Covercrops may be grown to improve the physical texture of the soil and 
thus increase the rate of water penetration. Covercrops are also important 
to prevent soil erosion in orchards planted on even moderately sloping land. 
Legume covercrops may add nitrogen to an impoverished soil but the amount 
of nitrogen added will not always obviate the necessity of adding fertilizer. 
The water-holding capacity of the surface soil is at best only temporarily 
affected by the addition of reasonable amounts of organic matter. 

Organic Content of Soil. — The increase in the organic content of the soil due 
to covercropping is exceedingly small and temporary. First, the amount of 

"',.■ - 

Fig. 5 — A covercrop of melilotus clover (Melilotus indica) ready to plow 
under in late March. (From Bui. 332.) 

material actually added is small in proportion to the weight of an acre-foot of 
soil ; and secondly, the rapid loss by decomposition is constantly reducing the 
amount present. The decomposition of the organic matter is of greater impor- 
tance than its accumulation. 

Partly grown, immature plants contain a larger percentage of nitrogen than 
plants which are mature. The more mature a covercrop is before being incor- 
porated in the soil, the longer the period necessary for it to decompose because 
of the relatively lower nitrogen content as growth progresses. A covercrop, 
when allowed to mature fully and actually go to seed before it is worked into 
the soil, is usually a real setback to a walnut orchard. 

Many cases have been observed where a misuse of covercropping has appar- 
ently done much more harm than good. This, however, should in no way dis- 
courage the proper use of a covercrop. 

Covercrop Management. — Plants suitable for covercropping purposes and 
amount of seed to sow per acre are shown below : Pounds 

Crop per acre 

Melilotus clover 20 

Purple vetch 40 

Mustard 5-10 

Walnut Production in California 15 

Such legume crops as vetch or melilotus clover (also commonly called bitter 
or sour clover) make a relatively slow growth (fig. 5) and should be planted 
as soon after walnut harvesting as possible. Crops like mustard and rape grow 
more rapidly in the winter season and may produce a relatively heavy tonnage 
in only 90 to 100 days. In order to secure a good growth of mustard covercrop, 
it will be necessary in many orchards to fertilize with a nitrogenous fertilizer. 

Water Requirements of a Covercrop. — One of the important factors in 
growing a covercrop is water. In districts of low rainfall, one or more irriga- 
tions may be necessary to replace the water used by the covercrop. 

Climatic and seasonal conditions, as well as the length of the growing period, 
have a direct effect upon the total amount of water a covercrop will use. 


Good irrigation practice consists in replacing with as little loss as possible 
that moisture which is removed from the soil by growing plants. With this defi- 
nition to work from, it becomes apparent that to follow a satisfactory irriga- 
tion program for walnuts, some knowledge is required as to when, from what 
part of the soil, and in what amounts the trees take water. The relation of 
quality and quantity of crop produced and responses in tree growth to the irri- 
gation practice must also be considered, as well as the relations of soil type and 
climate to the amount and frequency of irrigation. 

Factors in Irrigation Practice. — In a discussion of the use of water by wal- 
nut trees, consideration should be given to the following: (1) total seasonal 
use, (2) use during different periods of growth, (3) apparent root distribu- 
tion, (4) relation of soil type to frequency and amount of irrigation water, and 
(5) effect of insufficient irrigation on tree growth and size and quality of nuts 

Field tests indicate that climatic factors and size of trees affect the total sea- 
sonal water requirements. Medium-sized trees grown in a climatic area typical 
of the warm interior sections, use approximately the same amount of water as 
large mature trees in the cool coastal belt. Orchards in an interior section 
require annually nearly twice as much water as do trees of the same size and 
age in the coastal area. Trees in the intermediate area use about 30 per cent 
more water than trees of a similar size and age in the coastal area. These varia- 
tions are attributed to differences in both summer temperature and humidity. 
Approximately one half of the total seasonal water requirement occurs in the 
months of July and August. 

To irrigate a walnut orchard properly, it is not enough to know merely the 
total seasonal use of water by the trees. The probable root distribution has a 
definite bearing upon the irrigation program. The normal distribution of wal- 
nut roots under various soil conditions has been determined by irrigation stu- 
dies which have traced the loss of soil moisture at different depths. Many 
observations lead to the belief that soil-moisture control to a depth of 9 feet is 
sufficient to maintain a walnut orchard in good condition. Approximately 80 
per cent of the soil moisture used by the tree to a depth of 9 feet during the 
growing season is taken from the upper 6 feet of the soil. 

In heavier soils a greater proportion of the water withdrawal from the up- 
per 6 feet is concentrated in the first 3 feet; in the lighter soils it may be more 

16 California Experiment Station Circular 364 

nearly equal in the 0-3 and 3-6 foot depths. The variations in soil of the first 
6 feet do not, however, seem to affect the water withdrawal from the 6-9 foot 
depths ; regardless of the nature of the surface soils, the 6-9 foot depths seem 
to account for only about 20 per cent. A sharply stratified surface soil and 
subsoil, or impervious layers of hardpan or rock, or a shallow water table will 
limit root activity to less than normal depths. 

Fall and Winter Irrigation. — Under interior valley conditions the soil dries 
out during the harvest period. Unless fall rains are unusually early and heavy, 
the trees may suffer from water shortage during fall months and fail to mature 
their wood properly. Tops are then subject to killing back by winter frosts. 

ggp-' v ... : 

-".' ^^(SPWi4SI^SH 

-J, /".i^g. 


R* y 1 -' 

-j>^H '"■■£ A'S'* ■*mt 

, -<J&2 - ,' />*SN-i£ 


rf .>~w i q^. wj&f^/QkwSti 


*-"* • '.« _'-' 

y * V ^f\3 

•'^^w^vJ^^ * -S^\G#^ • > 

l y 3 

^ l fi^O^,^™^^M^^^^m 





jJajB^flffvV*' r . * *•■+: *%* -* 

dP'X .' *- ' ." *-' £"•*'•"**»-':.'" 


'*2l1k^***-\ - ^vKrHS*3K5S}c^!5B 

B|E^P?it y3fV'* 

^ J*' "' v> ^*~- A>'' fci ®^«>a?Hi 

%*?■ r ^^^^^^^§^^l 

1 ^ 

V'& ''♦ ^ - . ' .s'^SHKbSHBII 

Fig. 6. — This system of the dike and check furrow irrigation is used to prevent 
runoff and obtain an equitable moisture distribution throughout the orchard. 
(From Bui. 332.) 

Therefore, an irrigation is needed immediately following harvest in most 

When winter rainfall is not sufficient to wet the soil to field capacity to a 
depth of at least 6 feet, irrigation water should be applied during late winter 
to make up the deficiency. It may be necessary to hold the water on the land 
for several days and to obtain deep penetration, as shown in figure 6. 

Soil Moisture in Relation to Size and Quality of Nut. — The special value of 
abundant soil moisture in the early spring is in producing large nuts. Win- 
ter irrigation to supplement a low seasonal rainfall may be looked upon as an 
insurance against winter drought and the resultant small percentage of large 
nuts. The walnut shells of most of the varieties begin to harden about the mid- 
dle of June, after which there is little or no increase in size. 

The most rapid growth of the nuts takes place during the 5 or 6 weeks 
immediately after the blossoming period. A shortage of soil moisture in a wal- 
nut orchard during the early part of the growing period will result in produc- 
tion of a large percentage of small sizes. No amount of midsummer or late 
irrigation water will increase the size after the walnut shell hardens. 

Walnut Production in California 17 

Winter irrigation, when rainfall is insufficient, supplies moisture for the 
spring growth of the. trees. Small amounts of moisture only are transpired by 
the dormant trees or lost through soil surface evaporation. 

The effect of irrigation practices on quality of the kernels may be summed 
up as follows : the lack of soil moisture during June, July and August results 
in poor quality, as measured by the relative plumpness of the kernels. 

Irrigation Methods. — On fairly flat lands walnuts are usually irrigated by 
means of furrows, contour checks, strip checks, square checks, or a combina- 
tion check and furrow system. The check systems are better adapted to the use 
of relatively large heads of water. On steep slopes the contour furrow or ordi- 
nary furrow system is used. 

For a working knowledge of the effective irrigation of a deep-rooted crop 
like the walnut, it is necessary to sample the soil at various depths. Only by 
the use of a soil tube or auger can the farmer more than guess the soil-moisture 
conditions throughout most of the root zone. 

A knowledge of the amount of water used is as important as studying its 
movement through the subsoils. It may prove to be a greater mistake in the 
end to apply too much water than not to apply enough. This is especially true 
with stratified soils, or where a water table exists. 

The actual method of applying the water to the soil must be adapted to the 
soil conditions, grade of the land, and other local factors in each case. 


Intercropping of young walnut orchards is the general practice. It has 
usually proved successful in making the land support the orchard before the 
walnuts come into bearing. 

Annual Intercrops. — The intercrop to be grown will depend upon various 
circumstances. In the bean-growing districts, beans are an ideal intercrop for 
the young orchard. Beans are often grown in an orchard until the trees are 
eight to ten years old. 

Outside the bean-growing districts, the choice of intercrops depends largely 
upon the market conditions for the proposed crops. Several of the vegetables 
commonly used for canning may be grown in the young orchard without 
harm to the trees. Such crops as tomatoes, peppers, and melons are often used. 

Intercrops of corn, milo, squashes, and pumpkins are not generally grown, 
because of their apparently harmful effect upon the trees. Alfalfa is occasion- 
ally used as an intercrop, but is not recommended. 

Fruit Trees. — Peaches, prunes, and apricots are among the fruit trees most 
commonly used in intercropping. In many instances young orchards inter- 
planted with these fruits have made less growth than orchards interplanted 
with beans. Nevertheless, in many instances, the fruit trees have been profit- 
able and have made the orchard self-supporting while the nut trees were grow- 
ing. If fruit trees are used as an intercrop, the farmer must consider the 
irrigation problem carefully to be certain that the needs of the walnut trees 
can be harmonized with those of the companion crop. 

18 California Experiment Station Circular 364 


Field trials with various kinds and amounts of fertilizers show that nitrogen 
fertilizers are essential in most walnut orchards to maintain productivity. 
There are three sources of nitrogen for walnut trees : (1) the native nitrogen in 
the soil, (2) the nitrogen furnished by legume covercrops, and (3) the nitro- 
gen added to the soil by manures, hays, or chemical fertilizers. 

In most walnut orchards the native nitrogen supply in the soil is either so 
little, or its conversion into nitrate nitrogen is so slow, that the trees eventually 

Fig. 7. — Effect of fertilizer on mustard covercrop : area in foreground not 
fertilized; that in background received 100 pounds nitrogen per acre. 

suffer from nitrogen deficiency. In some instances this may occur before the 
trees come into bearing. In others, it may not be evident for a long time. 

While it is true that legume covercrops do add small amounts of nitrogen to 
impoverished soils, they do not add enough nitrogen to supply the needs of a 
producing walnut orchard. Consequently, nitrogen from manures or fertilizers 
is necessary to make up this deficiency in the soil. The main objections to the use 
of manures and hays are that they are slow in action and in some districts are 
relatively costly. Fertilizers, such as nitrate of soda, ammonium sulfate, am- 
monium nitrate, calcium nitrate, ammonium phosphate, and anhydrous am- 
monia, are all good sources of nitrogen, and their cost per pound of actual 
nitrogen is relatively low. In general, the one which provides the most nitrogen 
for the least money should be used. 

A tree affected by extreme nitrogen deficiency has sparse, yellow, and small 
leaves. There are also many dead twigs in the top of the tree. There are two 
other characteristics usually prevalent if the trees are receiving inadequate 
nitrogen. The first is a gradual failure to produce good covercrops, especially 

Walnut Production in California 19 

of the nonleguminous varieties, such as mustard. The second is a slow but 
steady decline in yield. When either or both of these symptoms are apparent, 
nitrogen should be applied as a trial in part of the orchard. 

One of the usual effects of fertilizing walnut orchards is the increase in the 
growth of covercrops. This is strikingly shown in figure 7 ; the mustard cover- 
crop in the foreground received no fertilizer, while that in the background was 
fertilized at the rate of 100 pounds of nitrogen per acre. 

The amount of fertilizer which can be used most profitably will vary wide- 
ly, depending, among other factors, upon the native fertility of the soil, the 
age of the orchard, and the past farming and fertilizing practice. Where heavy 
annual covercrops are grown, it has been generally considered profitable to 
use moderate to heavy applications of 100 or more pounds of nitrogen per 
acre per year. The advantage of this practice will obviously depend also upon 
the price of fertilizer and the value of the increased production of nuts. 

Zinc Deficiency. — Zinc deficiency, also called "little-leaf," occurs in some 
orchards on sandy soils in the San Joaquin Valley and interior districts of 
southern California; there are cases in the Sacramento and Salinas valleys 
also. For some obscure reason it is often found in spots where corrals were for- 
merly located, regardless of soil type. 

In very bad cases there is no normal foliage ; only small, yellow leaves are 
produced, the twigs die back from the tips each year, and the trees are thus 
badly stunted. In less severe cases trees make good shoot growth in early 
spring, followed by yellowing and curling of leaves in June and later in the 
summer. Some good growth may be produced, however, in late summer. Many 
of the leaves are mottled with light-green patches between veins. 

Affected walnut trees respond to a treatment consisting of the insertion of 
pure zinc or galvanized iron pieces into the sapwood. A glazier's point driver 
is a satisfactory tool for inserting pure zinc glazier's points into trunks or limbs 
up to 2 inches in diameter. These points should be driven well into the sapwood, 
each piece turned parallel with the grain. They should be scattered around, 
and spaced 1 inch or more apart at the rate of 30 per inch of diameter of limb. 
For treatment of larger trees, rectangular, galvanized iron pieces about % inch 
wide and 2 inches long, cut from 22- or 24-gauge material, can be driven into 
trunks or branches not larger than about 10 inches in diameter. They likewise 
should be driven well into the sapwood, parallel with the grain, and spaced in 
rings around the trunk or limb with individual pieces about 2 inches apart, in 
a staggered pattern. The rings should be about 2 inches apart, Usually at least 
six rings are needed. This gives a minimum of three pieces for every inch cir- 
cumference but spaces the pieces about 2 inches apart in all directions. The 
work can be done at any time of year. Usually one treatment will provide suf- 
ficient zinc to keep the trees healthy for several years. 

Soil treatments are sometimes satisfactory, but because of variation in the 
fixing power of different soils, the zinc may become available very slowly ; gen- 
eral recommendations of amounts and frequency of application are therefore 
unreliable. Best results have been secured in very sandy soils with a low fixing 
power. Zinc sulfate can be placed in a trench about 4 to 6 inches deep and about 
2 feet out from the trunk. Amounts to use per tree vary from about 5 to 10 
pounds for trees 2 to 8 inches in diameter, to as high as 30 to 50 pounds for 

20 California Experiment Station Circular 364 

large trees. Material should be applied in early winter. Spray treatments of 
zinc have not been satisfactory on walnuts. 

Deficiencies of Manganese and Copper. — Manganese deficiency is most com- 
mon in the coastal counties of southern California, but is also found to a limited 
extent in interior districts of central California. It is characterized by light- 
green color between the veins of leaves, darkening of a few scattered small 
leaf veins, and scorching of irregular areas in the leaves between veins and 
extending to the margin. In severe cases leaves are badly scorched and many 
drop, beginning in midsummer and increasing in late summer. 

Satisfactory results have been secured by spraying with about 5 pounds of 
manganese sulfate in 100 gallons of water just before the first leaves are fully 
developed — in May or early June in southern coast counties. 

Copper deficiency is found in limited areas in coast range valleys and moun- 
tains of central California. Walnuts from affected trees have very badly shriv- 
eled kernels. Shoots of the current season develop dark spots in the bark late 
in the summer; leaves near tips turn yellow and drop, and many shoots die 
back. Spraying of leaves with bordeaux mixture, 12-6-100, in early summer 
has given good results in preliminary trials. Soil treatments of 10 to 20 pounds 
of copper sulfate per tree in trenches 2 feet from the trunk have shown some 
results, but the effects are subject to variations in soil fixing power for copper. 


Two methods of pruning walnut trees prevail in general practice. In one, 
the pruning consists merely in the cutting off of the lower limbs that interfere 
with cultural practice. In the other, the trees are thinned throughout the tops 

The fact that few walnuts are produced in the centers of the old trees has 
suggested a moderate thinning out of the branches from year to year in an 
attempt to promote production more uniformly throughout the trees. Sunlight 
is necessary for the production and maintenance of fruit spurs, and without 
some thinning out practically all of the crop is produced on the outside twigs, 
in the tops, and on the sides of the trees. 

In all pruning operations, the limbs should be cut off without leaving any 
stub. All wounds larger than 3 inches in diameter should be painted with a dis- 
infectant, such as thin bordeaux paste ; this should be allowed to dry, then 
covered with a weatherproof paint, because walnut wood decays rapidly. 

Old orchards in which the trees are planted very close together should not 
be severely pruned to prevent the interference of branches from adjacent trees, 
but the number of the trees per acre should be reduced, the orchard thus being 
thinned out. The production in crowded orchards declines prematurely. After 
three to five years a thinned orchard may be expected to return to normal 

Gradual elimination of temporary trees in orchards where trees are begin- 
ning to crowd is sometimes desirable. Side branches can be removed from the 
temporary trees, with one or more central upright limbs left to produce for sev- 
eral years. At least half of the bearing area of the temporary trees should be 
removed ; otherwise, the permanent trees are not likely to receive much benefit. 
For such pruning, the usual care in making cuts is not necessary. 

Walnut Production in California 21 

Satisfactory yields are associated with maintenance of good vigor in mature 
walnut groves. Average length of growth of at least 8-12 inches on upright 
shoots over the top of the tree seems desirable in interior districts. In coastal 
districts possibly 4-6 inches is a desirable minimum. Trees making only 1 to 2 
inches of tip growth usually show declining yields. Poor growth and dying of 
fruit wood are signs of declining vigor, and efforts should be made to determine 
the cause in each case. In many instances weak trees have responded to applica- 
tions of nitrogen. Improved irrigation practice has helped where trees needed 
more water. In some cases lack of vigor has been traceable to such specific causes 
as accumulation of injurious salts in the soil or attacks of scale insects. In a few 
orchards detailed fine pruning of fruiting wood has given beneficial results. 


The codling moth and the walnut aphid, which are widespread in California, 
are the major insect pests of the walnut. The walnut husk fly is a major pest in 
several counties, and should it become generally distributed in all the areas 
where susceptible varieties are growing, it might become more serious than any 
other pest. 

Codling Moth. — This insect passes the winter as a caterpillar, or larva, in a 
cocoon, in crevices of the bark, in pruning scars, in similar places on the 
trees, and in debris on the soil or actually in the soil itself. It is also found in 
drying trays, in dehydraters, and in buildings where walnuts have been tem- 
porarily housed, and in burlap sacks used in harvesting. In the spring, the 
larvae transform into pupae from which adults issue 18 to 30 days later. Emer- 
gence is prolonged over a period of from 6 to 8 weeks. The moths are active at 
dusk and, provided the temperature is approximately 60° F or higher with 
little or no air movement, females deposit eggs at this time. 

The first eggs of the season are laid on the green twigs and leaves, but later 
they are deposited on the nuts. Upon hatching, the larvae usually burrow into 
the nut and feed on the kernel. The earlier larvae of the season usually enter 
through the blossom end, or calyx, causing most of the nuts so infested to drop 
before reaching maturity. Later, entrance usually is made where nuts in clus- 
ters are in contact. When the shell becomes hardened, the larvae cannot pene- 
trate it directly and, hence, must gain entry to the kernel through the fiber at 
the stem end (fig. 8). 

From 35 to 45 days are required for the larva to become fully developed ; 
after this period, it usually leaves the nut and spins a cocoon in some such loca- 
tion as already indicated. In most of the infested localities there is one full gen- 
eration, a partial second, and sometimes a smaller partial third, according to 
seasonal climatic conditions. In some areas in northern California there are 
usually two full generations each year. 

The degree of infestation of the codling moth has increased in many sections 
of the state in the past three decades. Where control measures have not pre- 
viously been practiced, they should be undertaken when the packing-house 
records sho w 2 per cent or more of infestation in a normal-sized crop. 

5 The scientific names and further description of these insects are given in : Essig, E. O., 
and W. M. Hoskins. Insects and other pests attacking agricultural crops. California Agr. 
Ext. Cir. 87:1-197. Eevised 1944. 

22 California Experiment Station Circular 364 

The method of control is spraying with basic lead arsenate. Acid or standard 
lead arsenate has caused injury to the foliage and should not be used. How- 
ever, investigations are under way in northern California to develop a safe 
method for its use. The formula for the spray is as follows : 

Basic lead arsenate 4 pounds 

Deposit builder 1 pint to 1 quart 

Water 100 gallons 

Fig. 8. — A, Appearance of green nut after codling moth 
larva has entered on open side. B, Larva usually enters 
green nut on the stem end as here shown. C, A round or oval 
opening on the stem end of the cured nut is an almost cer- 
tain sign of infestation. Close examination will usually re- 
veal a certain amount of staining and, frequently, some 
f rass. D, Section of walnut showing the codling-moth larva 
and the damage it has done. (From Bui. 332.) 

The spray should be thoroughly applied, covering all of the nuts on the tree, 
with equipment that will insure good agitation and high pressure (500 to 600 
pounds). A tower similar to the one shown in figure 9 should be used when 
spraying trees 20 feet or more tall. Fifty to 60 gallons may be required for 
large trees. The time for applications depends principally on moth activity, 
which may be best determined through the use of bait pans hung in the trees 

Walnut Production in California 23 

Pans 8 to 10 inches in diameter, containing the proper fermenting bait, sus- 
pended in the upper one third and outer part of the tree attract moths when 
they are active and depositing eggs. The bait used is as follows : 

Malt (Diamalt) 1 pint 

Water 19 pints 

Yeast (compressed) 1 cake 

The bait pans should be put out when the walnuts begin to set, and should 
be kept in the trees until the majority of the spring brood of moths has 
emerged. A dozen bait pans, spaced about the orchard, will be sufficient. In 

Fig. 9. — Tower spraying for control of codling moth. 
The platform on which the spraymen are working is 30 
feet above the ground. (From Bui. 379.) 

most of the infested areas, one spray treatment thoroughly applied and prop- 
erly timed usually affords satisfactory control. However, in the Stockton area, 
on Payne walnuts heavily infested, two treatments are apparently necessary 
for good results. 

Walnut Aphid. — The walnut aphid ranks second to the codling moth as a 
pest. One generation of the aphid succeeds another in the spring and early 
summer until a heavy population is built up. 

Injury is caused by the aphids' feeding in great numbers on the sap of the 
leaves and giving off honeydew in which a sooty-mold fungus grows. As a 
result of heavy infestations, many leaves drop during midsummer. Conse- 


California Experiment Station Circular 364 

quently, there is less protection against sunburning of the nuts, and the trees 
are devitalized. The latter factor reduces the possibilities of a good crop of 
walnuts the following year. Furthermore, there is an increase in the per- 
centage of perforated shells and shriveled and dark-colored kernels. 

Where aphid control is necessary at the time of codling moth treatment, nico- 
tine sulfate at a dosage of % pint per 100 gallons may be combined with the 
codling moth spray. In certain seasons, it is necessary to treat for aphids before 
the codling moth application and again later in the season ; treatment is some- 
times necessary in districts where the codling moth is not a pest. For such treat- 

Fig. 10. — Dusting to control the walnut aphid. 

ment, dusting is recommended as shown in figure 10. Twenty-five to 30 pounds 
per acre of a 3 to 5 per cent nicotine sulfate dust is used. 

Walnut Husk Fly. — The walnut husk fly occurs in several thousand acres of 
walnuts in southern California. The Eureka, Franquette, and Payne varieties 
are most susceptible to attack. The Placentia, Ehrhardt, Neff varieties, and 
most seedlings are not usually infested to an important extent except in the 
oldest infested areas. 

The adult flies begin to emerge from the soil late in June or in July and con- 
tinue over a period of 6 to 8 weeks. The eggs are deposited in the green husks 
of the nuts, and hatch into maggots which tunnel through and feed upon the 
inner husk tissue. As a result, a soft decay develops which permanently dark- 
ens the shell of the nut. Such affected nuts become culls. The larvae spend 
about 35 days in the husk, at the end of which time they drop to the soil and 
burrow to a depth of several inches, where pupation occurs and where they 
remain until the following year. A few may remain for a period of two to four 
years before adults emerge. 

Walnut Production in California 25 

Cryolite, according to the formulas below, applied as a dust or a spray, is 
the most satisfactory treatment. 


Cryolite 35 pounds 

Petroleum oil (approximately 100 seconds viscosity) 2 pounds 

Frianite or tale 63 pounds 

For mature trees apply 3 pounds of the dust mixture per tree. This should be 
applied from two sides and, preferably, at night under quiet air conditions. 


Cryolite 3 pounds 

Water 100 gallons 

For large mature trees, such as the Placentia variety, apply about 30 gallons 
of the spray mixture. 

Two applications are necessary, the first when emergence of adults from the 
soil is occurring regularly, and the second about 30 days later. Since control 
on the most susceptible varieties has become more difficult in recent years, the 
first treatment is applied as a spray and the second as a dust. On less suscepti- 
ble varieties both treatments are usually applied as dusts. Dust treatment is 
less expensive than spraying. To insure satisfactory control, treatment should 
be given to all trees and other interplanted vegetation within the walnut grove, 
and also to a border zone 50 feet wide of whatever vegetation happens to be 

Bed Spider. — The red spider, or mite, is sporadic in occurrence as a pest on 
walnuts and is not generally widespread. It occurs more commonly in the dry 
interior valleys during midsummer and late summer and is usually most 
severe on trees where soil moisture is deficient. 

Dusting with DN dust D8 at the rate of 3 pounds per mature tree is the treat- 
ment most commonly employed. Subsequent treatments may be necessary. 

Minor Pests. — The following insect pests have periodically infested walnuts, 
but they are of less economic importance : Catalina cherrj^ moth, walnut blister 
mite, calico scale, frosted scale, brown apricot scale, Italian pear scale, walnut 
scale, and California red scale. Among the chewing insects occasionally infest- 
ing walnut foliage are the June beetle, the Fuller's rose weevil, and several 
species of caterpillars. For information on importance and control of these 
uncommon pests, the farmer should consult the county farm advisor or the 
local county agricultural commissioner. 


Walnut Blight. — The walnut blight is the most destructive disease affecting 
the walnut crop. The prevalence of this disease varies from year to year, and 
is usually worse in the coastal districts than in the inland valleys in southern 
California. The converse of this is true in central and northern California. 

Losses are often very severe, especially with early-leafing varieties, in years 
of prolonged spring rains or fogs which keep the trees wet all day. 

Blight is a bacterial disease which attacks the young and tender growth and 
spreads to the more mature wood, causing the affected areas to turn black and 
die. Under favorable conditions, the disease is especially destructive to the 
nuts. It causes them to turn black and drop off when % to i/g inch in diameter. 

26 California Experiment Station Circular 364 

It causes full-sized nuts to become blanks, and may make full-grown nuts un- 
marketable, except as culls, by reason of staining the shell. It shows on the nut 
as black spots most prevalent at the blossom end, but often scattered over the 
entire surface. 

Studies of the walnut blight organism, methods of carryover and distribu- 
tion, and control measures are still in progress. Spraying the trees with bor- 
deaux mixture just before the pollination period has given satisfactory control 
in some districts. In other locations, especially in older groves of less vigor- 
ous trees, bordeaux spraying has been discontinued because of injury to the 
foliage, lowered quality of walnuts, and faster build-up of aphids on sprayed 
trees. Copper oxide is less injurious to trees and quality of walnuts but seems 
slightly less effective in controlling the disease. Recent incompleted tests with 
ammoniated copper carbonate sprays are encouraging. 6 

Melaxuma. — This troublesome disease of the walnut tree has occurred in 
serious outbreaks. Studies of this disease were carefully made and reported in 
1914-15 by Fawcett. 7 The nature and treatment of the trouble may best be 
summarized by quoting from this author : 

Because of the oozing of dark watery material to the surface of affected areas, this dis- 
ease is often confused, under the name of "black sap," with sunburn, frost injury, injuries 
to the bark in cultivation, injury from the decay of wood at places where limbs have been 
cut off 

The most common location for Melaxuma cankers is at the crotch of the tree where the first 
limbs join the trunk. The first evidence of the disease is often a black area on the otherwise 
grayish bark which looks like a dab of tar. . . . The diseased area later becomes slightly 
sunken, shrinks and cracks. The "black sap" then oozes out in considerable quantities and 
stains the bark as it runs down the limb or trunk. The wood underneath is discolored for a 
short distance and this discoloration usually extends beyond the margin of the killed bark. . . . 

Cut out the cankers that have not gone too far on the trunk and larger limbs and dis- 
infect the wounds thus made. The dead and discolored bark should be cut away, getting a 
little beyond the margin of dead tissue. . . . Probably one of the best disinfectants to apply 
to the wound is the Bordeaux paste. ... If the canker has practically girdled the limb, the 
limb had better be cut out. 

Croivn Gall. — Crown gall, which is caused by a bacterium, is found on many 
of the deciduous fruit trees. This same organism causes crown gall on walnut 
trees. The gall is a large overgrowth which usually occurs on the crown and 
rarely develops higher on the tree. This overgrowth is rough and somewhat 
spongy, as shown in figure 11, and is not easily confused with the normal over- 
growth which often occurs at the bud union of the walnut trees. The galls grow 
slowly in size and in some cases the trees are girdled after several years. 

Galls should be eliminated before they get large. Small galls can be cut out 
and the wounds, as well as the tools, disinfected. Galls of any size can be 
treated without skillful surgery, by painting with a mixture of sodium-dinitro- 

6 For further details see: Eudolph, B. A. Bacteriosis (blight) of the English walnut and 
its control. California Agr. Exp. Sta. Bui. 564: 1-88. 1933. (Out of print.) 

Rudolph, B. A. A new blight control spray. Diamond Walnut News 22 (2) :4-6. March, 

Scott, C. E. Walnut blight today. Diamond Walnut News 26 (2) :4, 5. March, 1944. 
Ark, P. A., and C. E. Scott. New blight spray proves promising. Diamond Walnut News 

27 (2) :10, 11. March, 1945. 

7 From: Fawcett, H. S. Melaxuma of the walnut, Juglans re<)ia. California Agr. Exp. Sta. 
Bui. 261: 129-48. 1915. (Out of print.) 

Walnut Production in California 27 

cresolate suspension (Blgetol), 20 per cent, and wood alcohol, 80 per cent, 
by volume. Most of the hard outer part should first be removed from old galls 
by knocking it off or roughly cutting it off. The surface to be treated should be 
clean. Very large galls should not be treated completely at one time because of 
danger that the tree will absorb too much of the chemical. Inspection, retreat- 
ment where needed, and smoothing of the surface to induce more rapid healing 
over are important. Treatment is not practical if the tree is already almost 

if .1BH 

Fig. 11. — A long-standing case of crown gall. The gall usually is inconspicuous on 
black walnut rootstocks. (From Bui. 379; photograph by C. O. Smith.) 

Crown Rot. — Crown rot occurs on the black walnut when used as a rootstock 
and it usually occurs on trees located in poorly drained soil, or where excessive 
surface water has periodically collected. The disease involves the bark just be- 
low the surface of the ground in the region of the crown, extending frequently 
a short distance onto the lateral roots. In well-advanced stages, the taproot is 
killed, caused as often by a girdling effect at or below the crown as by the down- 
ward development of the disease. 

The first indications of infection — so far as the general appearance of the 
tree is concerned — are a somewhat stunted appearance of the tree, sparse foli- 
age, lack of twig growth, and a yellowish tint in the foliage. In more advanced 
cases, the trees appear drought-stricken in midsummer, and the leaves turn 
yellow and drop off, leaving a crop of nuts exposed to shrivel and later to fall. 
Such trees usually die the following spring. 

The lesion on the crown is characterized by a soft, black, frequently spongy 
and decayed condition just below the bud union and under the ground. The 
top margin of the diseased area usually stops at the line of the bud union. 

28 California Experiment Station Circular 364 

Crown-rot disease ordinarily may be controlled by either of the two follow- 
ing treatments : 

1. Complete drying-out of the crown seems to give good control of this dis- 
ease. Therefore, both as a preventive and cure, all trees on black walnut root- 
stocks should have the crowns exposed at all times, and irrigation water should 
never be run closer than 4 feet from the tree crown. 8 

2. After all lesions of any size and depth have been cut out, the wounds are 
painted with cyanide of mercury (1 part in 500 of water). Then the wounds 
are painted with a good waterproof paint. 

Twig Wilt. — Twig wilt is a serious new disease attacking walnuts in the hot- 
ter parts of interior valleys. The disease appears to be caused by a fungus. It 
attacks the bark and sapwood of twigs and branches, eventually plugging the 
conductive tissues and causing sudden wilting of leaves beyond the point of 
attack. Wilting usually occurs during hot periods in the latter part of summer. 
The dead leaves hang tenaciously on such twigs into the winter, forming char- 
acteristic "flags" in the trees. The disease develops slowly in larger branches. 
Eventually the entire top of the tree is killed. 

Control measures so far have been limited to the cutting out of diseased twigs 
and limbs in an attempt to retard spread of the disease. It is advisable to cut 
affected limbs a considerable distance below the visibly killed area. 

Winter Injury or Dieback. — Winter injury, or dieback, of walnut is charac- 
terized by death of the tops of the trees. Such injury is usually first noticeable 
during the early spring. The most common causes of this type of injury are 
given below : 

1. Early autumn frosts kill the immature, growing shoots. Young walnut 
trees are especially subject to injury from this cause. Such frosts cause the 
foliage to die prematurely and in extreme cases kill the ends of the shoots. The 
small denuded branches are subject to further injury from fall and winter sun- 

2. Late summer and fall drought causes dieback in either young or bearing 
walnut groves. Trees experiencing this condition fail to make new growth in 
the spring, except from the trunks or main limbs. Fall irrigation of the walnut 
groves has been found to eliminate the injury from drought. The amount of 
irrigation water to be applied depends upon the type of soil, the amount of soil 
moisture present at the end of the harvest season, and the length of time re- 
maining before the rainy season normally begins. 


Effect of Harvesting Methods on Quality of Crop. — The natural drop of the 
nuts occurs with most varieties between early September and early November. 
In actual practice, harvesting is hastened by shaking the trees in order to ob- 
tain the highest possible quality. The trees are shaken from 2 to 4 times, the 
number and vigor of shakings depending upon climatic conditions and variety. 
In the cooler areas the kernel of the nut matures over a long period of time. The 
maturity of the kernel and the cracking of the hull occur at about the same 

8 For further details see: Barrett, J. T., and C. O. Smith. Walnut tree crown rot. Diamond 
Walnut News 8(5) :4, 5, 7. December, 1926. 

Walnut Production in California 29 

time; because of this, 3 to 4 light shakings are usually desirable in these areas. 
In the hot interior areas, kernel maturity precedes hull cracking. Thus, in these 
areas, earlier and more vigorous shaking usually results in better quality. For 
best quality, walnuts should be gathered, hulled, and dried immediately after 

Nuts left on the ground too long are subject to darkening of the kernels, and 
may be injured by fog or rain. The effect of rain upon the nuts may be entirely 
superficial if they are picked up and dried promptly. At best, they will be dirty 
and more or less stained, causing extra work of washing and making proper 
bleaching more difficult. The kernels of nuts long on the ground after a rain 
become moldy and the shells stained on the outside. If the stain is pronounced, 
it will be impossible to bleach out, and the nuts will become culls. 

A certain percentage of nuts will drop with the dried husks adhering. These 
are "sticktights," and, in their plumpness and in the appearance of their ker- 
nels, they are likely to be inferior to the clean nuts. There is usually a high 
percentage of blanks, and shriveled, moldy, and dark-colored kernels among 
the sticktight nuts. The percentage of sticktights is greater during seasons of 
abnormally high temperatures, when the nuts are sunburned and also when the 
husks are affected with blight. Trees which have suffered drought during the 
latter part of the growing period, or which are subject to the attacks of aphids 
or red spiders, or which for any other reason lose their leaves prematurely, 
produce a high percentage of sticktights and inferior nuts. 

A small percentage of nuts will mold after the hulls crack and before the nuts 
drop from the trees. Sticktight nuts are more likely to be moldy at the time they 
drop from the trees than those which drop free from their husks. The percent- 
age of moldy nuts will increase if sticktights are left on the ground for a week 
or 10 days. 

The commercial grades are determined by the percentage of edible kernels, 
the percentage of light-colored kernels, and the appearance of the shell. The 
effect of harvesting methods upon the color of the kernels can be as pronounced 
as its effect upon the percentage of moldy nuts. 

Early Harvesting and the Use of Mechanical Shakers. — The success of early 
harvesting methods, rapidly carried out, depends upon the efforts of each farm- 
er in giving the program trial under his own conditions. The program may also 
be expected to vary with the variety planted and with the climatic conditions 
in the different walnut-growing districts. 

During the past few years there has been a rapid development of mechanical 
shakers for walnuts, and several hundred shakers are now in use. The shakers 
are usually attached to tractors ; the principles of mechanical shaking are fairly 
well developed, but designs are not yet standardized. 

Substantial savings in labor have been made by use of mechanical shakers as 
compared with hand shaking. Mechanical shaking is most successful in interior 
districts where walnuts mature over a shorter period and where more vigorous 
shaking is required than in coastal areas. The mechanical shakers do a more 
thorough job, especially on upright limbs of tall trees. Use of mechanical shak- 
ers makes early and rapid harvesting more practical in many interior districts, 
particularly with the Payne, Eureka, Mayette, and Franquette varieties. 

Some growers have effected additional savings in labor in harvest operations 

30 California Experiment Station Circular 364 

by use of special catching devices mounted on wheels, or by catching walnuts 
on very large sheets of canvas. Cleaners for removal of leaves are used with 
such equipment. 

Careful preparation of the ground by dragging and rolling before harvest 
makes harvesting easier. Where a large proportion of the crop can be shaken 
at one time on a smooth ground surface, hand rakes can sometimes be used to 
advantage by pickers. 

Hulling. — When harvest operations are hastened in an attempt to produce 
high-quality nuts, the frequent shaking of the trees will probably cause a por- 
tion of the nuts to fall with the hulls still attached. If discretion is used in shak- 
ing the trees, the large majority of the nuts should be in such condition that 
they can be hulled. Most nuts with cracked hulls can be hulled. Some varieties, 
such as the Payne, Mayette, and Blackmer, frequently can be hulled by hand 
or by machine before cracking is visible. Hulling can be done by hand or the 
nuts can be run through a machine. The latter practice is the quicker and more 
economical of the two. A machine, however, will not hull nuts which cannot 
be hulled by hand. It is a labor-saving device only and should be used as such 
in speeding up the harvest, which, in turn, means a higher-quality crop. 

Ethylene Gas 9 as an Aid to Rapid Harvesting. — The greatest obstacle to 
rapid harvesting, once the kernel is mature, is the large amount of green -stick- 
tights that fall when the first shaking is done. In some of the interior sections 
of the state, maturity of the kernel will sometimes precede loosening of the hull 
by 2 to 3 weeks. The normally warm weather during this period between kernel 
maturity and hull maturity can and often does result in a greatly lowered qual- 
ity of the walnut kernel. The green sticktights can be water-sweated to remove 
the hull but the water-sweating process is not always satisfactory. The use of 
ethylene gas as a means of loosening the hulls on the green, sticktight walnuts 
is much more rapid and freer from difficulty than the water-sweating process. 
The ethylene gassing method has been successful only in warm interior dis- 
tricts of southern California. It is now used to some extent on Placentia and 
Eureka varieties in these districts. It causes undesirable darkening of veins on 
the surface of walnut kernels in some coastal districts. The ethylene gas method 
of treatment is briefly outlined as follows : 

1. The kernels must be mature. Immature nuts shrivel badly and are worth- 

2. The trees should be shaken as hard as possible and the green sticktights 
separated from the nuts that will hull. 

3. The green sticktights are placed in a bin which can be made airtight when 
desired. The bin must be equipped with forced-draft ventilation. 

4. Ethylene gas is injected at the rate of 1 cubic foot of ethylene to 1,000 
cubic feet of air space (air space is determined on the basis of an empty bin) . 

5. The temperature in the bin should range between 70° and 80° F. Temper- 
atures lower than 70° slow the process. Temperatures higher than 80° not only 
slow the process but also tend to darken the kernel. 

6. The bin must be ventilated by means of forced draft at least every 12 
hours. In some instances more frequent ventilation may be necessary. A sud- 

9 For details see : Sorber, D. G. The use of ethylene in the walnut harvest. Diamond Walnut 
News 21(5) :10, 11, 14. September, 1939. 

Walnut Production in California 31 

den rise in temperature within the bin is an indication that the bin should be 
ventilated. Ventilation may require from 20 minutes to 11/2 hours. 

7. The bin should be regassed after each ventilation period. 

8. Treatment should be continued until 96 to 98 per cent of the nuts will hull 
readily. This will require from 24 to 72 hours. The greener the hulls, the longer 
will be the time required for treatment. 

Washing. — Immediately after they come through the huller, the nuts must 
be washed to remove the juice of the crushed hulls, which would otherwise stain 
the shell and make bleaching impossible. The washing process will not remove 
the stain on the shells caused by sunburned or blighted hulls. 

Washing is done in a large, cylindrical drum made of a coarse wire netting 
in which the nuts are slowly revolved under a stream of water. They should 
pass through the drum in a continuous stream so that no individual nut will 
be in the washer more than 2 or 3 minutes. Nuts which have lain in the mud, 
even though they dropped to the ground free from the hulls, should also be 
washed ; otherwise, they will be difficult, if not impossible, to bleach. 


After walnuts are hulled they must be promptly and thoroughly dried in 
order to evaporate the excess moisture contained in both kernel and shell. 
Dehydration will (1) give a product of stable weight, (2) prevent further 
molding or darkening of kernels, and (3) permit efficient bleaching. 

The amount of moisture which must be evaporated varies greatly, but de- 
creases as the harvest season progresses. Adequately dried nuts average 6 per 
cent moisture, and should not contain in excess of 8 per cent. In recent years 
the sun-drying of walnuts has been largely superseded by the use of dehydrat- 
ers. The dehydraters have the following advantages : 

1. Dehydraters operate independently of weather conditions, and afford 
thorough and uniform drying in an average of 24 hours, arrest further molding 
or darkening of kernels, and minimize the splitting of shells. 

2. Labor requirements for dehydration are less than for sun-drying. Auto- 
matic heat control permits dehydraters to run with a minimum of attention. 

3. Since the peak of demand for walnuts comes in the fall before the normal 
peak of supply, any operation which speeds delivery of walnuts to the consum- 
er is valuable. Dehydraters afford a steady delivery of walnuts to packing 
houses for prompt packing and shipping to market. 

Principles of Dehydration. — The evaporation of moisture requires the ab- 
sorption of a definite amount of heat. In dehydration this heat is brought to 
the walnuts by a continuous stream of air previously heated in a furnace fired 
by stove oil, natural gas, or butane. 

The air conveys heat to the nuts and passes on to absorb and remove the 
water vapor evaporated by that heat. Natural air currents are neither adequate 
nor controllable ; consequently, fans of the centrifugal, multiblade type are 
used to blow a large volume of air through the spaces between the walnuts in 
the bin. 

Efficient dehydraters are provided with an air flow of 2,000 to 3,000 cubic 
feet of air a minute for each ton of walnuts. This air flow requires the expendi- 
ture of 1 to 1% horsepower. 

32 California Experiment Station Circular 364 

As the air passes through a bin of walnuts, its drying power rapidly de- 
creases because absorption of moisture causes a decrease in temperature and 
an increase in humidity. Even with vigorous air flow it has been found inad- 
visable to use a depth of walnuts greater than 2 feet if the air flows in one direc- 
tion only. Otherwise, the nuts on the intake side will reach dryness while the 
nuts on the exhaust side are still insufficiently dried. 

By periodically reversing the direction of the air flow, the depth of nuts can 
be increased to about 4 feet. When this is done, all nuts in the bin receive the 
same average air conditions and consequently dry at uniform rates. 

The capacity of any dehydrater in terms of pounds is measured by multi- 
plying the size in cubic feet by 20 pounds, the weight of a cubic foot of dried 

Walnuts have a critical temperature of 110° F. If heated higher the oil in 
the kernels will become rancid and make them inedible. Rancidity is not ap- 
parent immediately after overheating but requires from a few weeks to several 
months to develop. One lot of overheated nuts mixed with other lots of normal 
ones will result in the entire lot's becoming unsalable because the rancid nuts 
cannot be separated from the good ones. Because of the spoilage caused by 
overheating, every dehydrater used for walnuts must be equipped with an 
accurate and dependable thermostat which will automatically prevent the tem- 
perature from exceeding 110° F. The use of lower temperatures is not objec- 
tionable except that lower temperatures require a longer time for drying. 

Selection of a Dehydrater. — Dehydraters can be purchased from one of 
several reliable manufacturers, or can be built by obtaining plans and specifi- 
cations from the University of California College of Agriculture. Walnuts can 
be dried successfully on trays in standard types of air-blast dehydraters, such 
as are used for fruits or vegetables, provided they are equipped with a special 
thermostatically controlled heating system which does not permit the tempera- 
ture to exceed 110° F. 

Although the total cost of operation and maintenance of a dehydrater is 
slightly more than the corresponding cost of sun-drying, the advantages of 
this modern method of drying counterbalance its slightly greater cost. 

Sun-drying. — Sun-drying is accomplished by spreading the nuts in shallow 
trays with bottoms made of slats spaced about % inch apart. The nuts should 
not be left exposed to the sun during the entire day if the weather is especially 
dry and hot, for many of the nuts will crack open. If the trays are spread out 
in the morning and the nuts thoroughly stirred several times, the trays may be 
piled up when the nuts are still warm. 

The trays should be so piled as to allow ventilation between them. At night 
the nuts should be protected against exposure to fog or dew. The alternate loss 
and gain in moisture when the nuts are exposed to fog at night and sunshine 
in the day will cause many to split. 


Although the packing and selling of walnuts is distinct from their produc- 
tion, each grower should know how they are to be graded and packed in order 
to realize the importance of delivering high-grade nuts to the packing house. 

After being properly cured, the nuts are delivered to the local packing house. 

Walnut Production in California 


There they first pass under a vacuum hood, which removes the blank or imper- 
fectly filled nuts. From the vacuum, nuts with full kernels pass on to an endless 
belt where they are hand-culled to remove those which are obviously imperfect. 


A Standard of Costs for Walnut Production in a Mature Orchard in California 

Cost item 

Cost per acre 




Pruning , 

Brush disposal 

Planting of covercrop 

Dusting and spraying 

Disease control and surgery 

Cultivation, four times, one way 

Preparation for irrigation 

Irrigation, three or four times 

Miscellaneous, bracing, etc 

Cultural labor costs 

Tower work, knocking, etc 

Picking 1,500 pounds 

Hauling out, hulling, and dehydrating 

Hauling to packing house 

Harvest labor costs 


Irrigation water 

Covercrop seed 

Dust and spray materials 

Fuel and power for dehydrating , 


Material costs 

General expense, 5 per cent of total labor and material 

County taxes on orchard and equipment , 

Repairs to machinery and equipment 

Insurance: compensation and fire 

Cash overhead costs 

Total cash costs 

Depreciation on trees 

Depreciation on improvements and equipment 

Total cash and depreciation costs , 

Average farm income from 1,500 pounds per acre 

Average profit from 1,500 pounds per acre 



4.20 ) 
13.50 J 







































The nuts then pass through revolving drums containing a liquid bleach 
which is harmless to the kernels. Dirt and stains are thus removed and the shells 
made uniformly bright and clean. The walnuts next pass by on a sorting belt 
where experienced women pick out the nuts with imperfections that bleach- 
ing has revealed. Such nuts include those which are wormy, sunburned, or 

34 California Experiment Station Circular 364 

moldy. Next the walnuts are mechanically sized into the three standard grades : 
large, medium, and baby. Since the larger-sized grades bring considerably 
higher prices, it is obvious that cultural practices, such as cultivation, fertiliza- 
tion, and irrigation, that result in good sizes and grades are most important. 
Each size grade is run into large, thoroughly ventilated bins where the nuts 
remain until the moisture absorbed in bleaching is evaporated. From the dry- 
ing bins the nuts pass by on a third and final culling belt en route to being 
packed in bags or cartons. Most of the crop is packed in 100-pound bags for 
shipment. Since most of the walnuts picked out as culls have good kernels, all 
such culls go to shelling plants where the edible kernels are recovered and sold 
as shelled walnuts, while the inedible kernels and shells are converted into 
various by-products. 


Approximately 85 per cent of the walnut crop is packed and sold for its 
members by the California Walnut Growers Association, a noncapital, non- 
profit, cooperative composed of local associations in all walnut districts of the 
state. The board of directors of the central association is composed of one rep- 
resentative from each local association. The local associations process and pack 
the nuts delivered by their members in accordance with the standard estab- 
lished by the directors of the central association. Association members receive 
the market price of the particular grade of their delivery, less the cost of pack- 
ing and marketing. Walnut growers who are not association members usually 
sell their crops to commercial packers. 


Every grower endeavors to obtain as high a profit as possible; hence, he 
strives to secure a maximum yield of high-quality walnuts, to sell this yield at 
as high a price as possible, and to secure this production at as low a cost as 
practicable. Table 1 shows costs during 1934, 1941, and 1942, and is a standard 
with which a grower may compare his own costs. An economical system of man- 
agement, similar to that followed by efficient growers, has been assumed in pre- 
paring this table; it is shown in enough detail so that growers may use it for 
comparison with their own conditions. Harvesting costs and all costs per 
hundredweight are based on a yield of 1,500 pounds per acre. Perhaps some 
of the costs shown in this table are below average, but they are all within the 
range shown by growers' records and may well serve as an indication as to 
whether a grower's costs are unnecessarily high or too low to result in a satis- 
factory income.